The invention relates to an air-conditioning unit according to the preamble of claim 1.
The use of internal combustion engines that are optimized in terms of fuel consumption in motor vehicles also affects the climate control of said motor vehicles by the fact that, in certain operating ranges, e.g., during the starting phase when outside temperatures are low, the amount of heat given off to the coolant is not sufficient to comfortably heat the motor vehicle. Auxiliary heating units are therefore required in order to ensure comfort at low temperatures and to enable deicing of the vehicle windows, if necessary. An air-conditioning unit can also serve as auxiliary heating unit, especially since an increasing number of motor vehicles are being equipped with an air-conditioning unit as a standard feature. At low temperatures, the air-conditioning unit is used as a heat pump by reversing the flow of refrigerant. This consumes minimal energy and responds spontaneously with high heat output.
Additionally, the conventional refrigerant, R134a, a partially fluorinated hydrocarbon, is being replaced in air-conditioning units to an ever-increasing extent with carbon dioxide (CO2), a natural refrigerant. In air-conditioning units that use carbon dioxide as the refrigerant, heat is not given off by condensation of the refrigerant in a condenser, but rather at a supercritical pressure in a gas cooler. If this air-conditioning unit is used as a heat pump, heat is absorbed via the gas cooler. A substantial disadvantage of a heat pump of this nature, however, is the fact that the gas cooler ices over on the air side when outside temperatures are low. As a result, an inadequate amount of air flows through the radiator of the internal combustion engine, which is usually installed downstream in the flow of air, so that adequate cooling of the internal combustion engine is not ensured.
An air-conditioning unit for motor vehicles is made known in DE 198 06 654 A1, that uses carbon dioxide as refrigerant that circulates in a refrigerant circuit in a liquid or gaseous state, depending on the phase. A compressor pumps the refrigerant in a cooling operation through a gas cooler, a heat exchanger, an expansion device, an evaporator, and through the heat exchanger under low pressure back to the intake. In the gas cooler, the refrigerant gives off a portion of its heat that was generated by compression in the compressor. Said refrigerant transfers a further portion of its heat in an inner heat exchanger to the cooler refrigerant flowing back to the intake. In the expansion device, the refrigerant is expanded to a temperature that is below the ambient temperature, so that, in the evaporator, it can remove heat from the air that flows into the air-conditioning device, and it can dry said air simultaneously, if necessary. The air is then brought to the desired temperature using a heating heat exchanger installed downstream.
By reversing the flow distributors, the compressor pumps the refrigerant in a heating operation first of all through the evaporator, through which said refrigerant now flows in the reverse direction. The refrigerant gives off a portion of the heat generated by compression to the air flowing into the air-conditioning unit. Said air therefore warms the passenger compartment and deices the windows. After the evaporator, the refrigerant is then expanded in the expansion device to a lower temperature, so that it can absorb heat from the coolant circuit on its way to the intake of the compressor in a coupled heat exchanger that is located between the coolant circuit and the refrigerant circuit. Using an air-conditioner of this nature, it is possible to increase the temperature in the passenger compartment under cold weather conditions, at the expense of the internal combustion engine.
Furthermore, a device and a method for heating and cooling a passenger compartment of an internal combustion engine are made known in EP 0 945 291 A1. In the heating operation, the refrigerant is compressed by a compressor and travels through a 3/2-way valve to an evaporator, in which it gives off a portion of the heat produced by compression to the colder air inside the passenger compartment. From the evaporator, the refrigerant flows to an expansion device, in which it is cooled down to the point at which it can absorb heat from the ambient air at a gas cooler installed downstream. Additional heat is supplied to the refrigerant in an exhaust-gas heat exchanger installed downstream that is acted upon by hot exhaust gases from the internal combustion engine.
From the exhaust-gas heat exchanger, the refrigerant returns to the compressor once more, and the refrigerant circuit is closed. If the refrigerant in the expansion device is expanded to a temperature that is below the ambient temperature, the air flowing through the gas cooler can be cooled down to a temperature below the saturation temperature. In this case, water condenses out of the drawn-in ambient air. If the temperature is below the sublimation line of water, said water changes to the solid state, and the gas cooler ices over. Since the gas cooler is usually installed upstream of a radiator of the internal combustion engine as viewed in the direction of flow of air, proper cooling of the internal combustion engine is endangered when the gas cooler ices over. To prevent excessive icing over, a by-pass line is opened by way of a 3/2-way valve when ambient temperatures become critical, so that the gas cooler is closed briefly. The refrigerant bypasses the gas cooler and flows directly to the exhaust-gas heat exchanger and, from there, to the intake of the compressor.
Supercharging devices, referred to as “superchargers”, for internal combustion engines increase the air throughput by compressing the air required to combust the fuel, while the piston displacement and rotational speed remain the same, therefore making greater power density possible. The air heated by compression—also referred to as “boost air”—is cooled off before it enters the combustion chamber, which further increases its density and, therefore, the power of the internal combustion engine.
The boost air can be cooled in an intercooler by the coolant or by the outside air; the air-cooled variant has proven to be more popular on motor vehicles. The heat given off by the intercooler is dissipated, unused, into the environment.